Concentrate on armor that favors your class and try to avoid the "Dated" stuff (although some are still good and you might be able to get them cheaply). For equipment that has an Optimal Rank, I highly suggest you wait until you actually hit said rank or you will be taking a huge penalty trying to equip it earlier. Honestly though, levels 1-20 fly by so fast I wouldn't worry too much about it. Levels 20-40 is where you would want to invest gil in good armor or time in hunting NMs.